The present invention relates to a fully differential, variable gain amplifier comprising an input, an intermediate stage and an output stage, and voltage to current conversion means, the input stage being coupled to the intermediate stage and the intermediate stage being coupled to the output stage.
Wide band amplifiers are very useful devices in applications where stable and predictable parameters, gain among them, are required over a maximal bandwidth.
These applications include optical fiber transmitters and receivers, input amplifiers for amplifying the output signals in a read head of an optical player and many other applications.
These applications usually trans employ impedance Amplifiers (TIA). These amplifiers have resistive feedback means. Both the bandwidth and the gain of the TIA depend on the resistive value of these feedback means.
For many purposes, it is desirable to achieve a high gain and a maximal bandwidth. However, these objectives cannot be achieved at the same time with the standard TIA described above.
It is furthermore desirable that the TIA can work with both differential and single-ended signal sources in order to obtain a greater flexibility in applications, without their performance being reduced significantly.
As long as the previously presented approach is maintained, only some aspects of TIA performance can be improved at the expense of very complicated solutions, while it does not provide a gain which is controllable independently of the bandwidth.
An amplifier of the type described in the opening paragraph is disclosed in U.S. Pat. No. 5,581,212. In order to broaden its bandwidth, this amplifier has three stages with a TIA as the final stage. Again, (the operation of) this wide band amplifier cannot provide a gain which is controllable independently of the bandwidth of the amplifier.
It is therefore an object of the present invention to provide a fully differential, variable gain amplifier and a multidimensional amplifier arrangement with the possibility to controlling the bandwidth and the gain independently of one another.
In accordance with the invention, this object is achieved in a device as described in the introductory paragraph, which is characterized in that the input stage comprises an input differential amplifier having local feedback means and voltage-to-current conversion means, for coupling said input differential amplifier to the intermediate stage, and in that the intermediate stage comprises a first and a second branch from respective common nodes to a reference node, which common nodes are shared by said input stage, output stage and intermediate stage, the intermediate stage further comprising current controlled networks coupled to the common nodes via first feedback branches and to the input stage via second feedback branches. The local feedback means lower the input impedance and raise the cut-off frequency of the input differential amplifier, thereby widening the bandwidth. The second feedback branches further lower the input impedance and at the same time, raise the cut-off frequency of the input differential amplifier. The first and the second branch between the common nodes and the reference node may be implemented as low impedance branches, enabling the impedance of the common node to be reduced. A direct consequence of this is that the time constants associated with the common node decrease and the bandwidth increases. The connection between the output of the input differential amplifier, on which output voltage signals appear and the common nodes includes voltage to current conversion means to adapt said output signals to the low impedance nodes. Said common nodes are shared by the inputs of the output stage, wich reduces the input impedance and expands the bandwidth of this stage. It is clear that the bandwidth of the fully differential, variable gain amplifier, hereafter denoted as xe2x80x9cAmplifierxe2x80x9d, is controlled by means of said local feedback, the first feedback branches and the second feedback branches. Since said output stage is not involved in this system of feedbacks it can control the gain independently of the bandwidth of the Amplifier. This makes the Amplifier according to the invention particularly suitable for wide band applications.
The embodiment of the Amplifier as claimed in claim 2 has the advantage of an increased Common Mode Rejection Ratio (CMRR) with the direct consequence that the overall signal to noise ratio (S/N) is also improved.
Since the positive feedback is embedded in the negative feedback the stability of the Amplifier is improved.
By way of illustration, all the stages described hereinbefore are realized with transistors. In an embodiment all these transistors are implemented in bipolar and CMOS technology.
The low input impedance of the Amplifier according to the invention allows a very simple connection of a current source to its input. This is of particular interest for applications as optical receivers, where the input device is an optical detector. Usually these devices are implemented as a semiconductor photodiode that behaves as a current source.
It is another object of the present invention to provide a multidimensional amplifier arrangement characterized by a first, N1 dimensional array of Voltage Controlled Current Sources (VCCS array), a second, N2 dimensional array of fully differential, variable gain amplifiers (TIA array) as claimed in claim 1, and a Control Unit, said Control Unit being coupled to the VCCS and the TIA arrays.
The VCCS Array provides means to adapt a Voltage Input Vector to the inputs of any TIA in the Array. The Control Unit comprises means for routing the Input vector either to the VCCS Array or to the TIA Array. The Control Unit also comprises means to control either the size of the VCCS Array or the size of the TIA Array.
The above and other features and advantages of the invention will be apparent from the following description of exemplary embodiments of the invention with reference to the accompanying drawings, in which: